"Just having been infected with something - having that exposure - may permanently change something in our biology," says Ingram, who has carried out the research for her PhD in molecular and cell biology.

Ingram says previous research has found that when rodents are infected by the protozoan Toxoplasma gondii, they lose their aversion to the smell of cat urine.

To date scientists believe that this is because toxoplasma is disrupting the fear pathway, either by creating cysts and inflammation in the brain, or by producing dopamine to override it.

In her study on mice, Ingram found that different strains of toxoplasma also had the same fear-removing effect.

"We show that loss of innate aversion of cat urine is a general trait caused by infection with any of the three major clonal lineages of parasite," the researchers write.

But, more surprisingly, she found that even when the microbe appeared to have been cleared from the animal altogether the mice were still not afraid of cat urine.

By four months after the infection neither parasite nor ongoing brain inflammation were detectable and yet the behavioural changes were still observable.

Implication for humans

Toxoplasma is extremely widespread and can be caught from contaminated soil or water, or undercooked meat from secondary hosts, as well as from cats, which are its primary host.

"In cats it produces billions of infected spores in the cat gut and those get transmitted into the environment via the faeces," says Ingram. "The cat skats can contaminate soil and water and can get on vegetation as well. Those spores persist in the environment for many years."

While a third of humans are believed to be infected by toxoplasma, it is generally considered to be asymptomatic, unless it causes toxoplasmosis in the brain.

Toxoplasmosis can cause miscarriage or birth defect or even death in people who are immune compromised.

But, she says it is still unclear whether toxoplasma could subtly affect human behaviour.

"There may be other aspects of our neurobiology being infected but in such a subtle way that it hasn't been found to be clinical yet," says Ingram.

More importantly, she says, the findings that a transient infection of a pathogen can cause behavioural changes in the long term could challenge the way that we think about infectious disease.

"It's exciting and scary," says Ingram. "It could be not only important to know what we currently have but also to know what we have been exposed to in order to understand better where symptoms are coming from."